Chemical vapor deposition relates to the process of chemically reacting or decomposing a compound or compounds at elevated temperatures to deposit decomposition products on surfaces of particles or large objects. Pyrolytic carbon may be deposited on a surface by thermally decomposing gaseous hydrocarbons or vapors of other carbon-containing substances. Metal carbides may be deposited on a surface by thermally decomposing a mixture of vaporized metallic halides and vaporized hydrocarbon or by decomposing a metal-containing organic compound. Silicon carbide may be deposited on a surface by decomposing an organic silicon compound, such as methyl trichlorosilane.
Chemical vapor deposition is frequently used to coat relatively small particles, such as nuclear fuel particles, and is also used to coat massive substrates producing composite articles in which the substrate is protected by the deposited coating. In some instances, a mandrel may be coated, and the coating later removed from the mandrel to serve as the end product.
It has been found that the crystalline structure and the density of coatings are dependent on several, independently variable operating conditions of the coating apparatus employed. Among the factors influencing crystalline structure and density are the ratio of the total surface area within the chamber to the free volume within the chamber. If small particles are being coated, the surface to volume ratio may be adjusted according to the total amount of the particulate material supplied to the chamber.
Large articles have very small surface areas relative to the surface area of an equal mass of tiny particles, and the article itself may not have sufficient surface area to provide an optimal surface area to volume ratio within the coating chamber. To adjust the surface area to volume ratio within a chamber in which a large article is coated, it is known to include within the coating chamber particulate material that is fluidized by the flowing gases.
Problems have developed in attempts to coat certain large articles in a chamber together with a fluidized particle bed. In particular, broad, thin articles may be broken by particle surging forces. Certain proposed solutions to such a problem, such as reducing the rate of gas flow through the coating chamber, may not be consistent with achieving the desired density and crystalline structure of the coating.
One attempted solution to the problem of article breakage was to support the suspended article at several points; however, this solution would be unsatisfactory for many coating applications because, wherever the article is supported, coating does not occur. Furthermore, fixtures for supporting articles at multiple points are complex, and the fixtures are themselves subject to breakage by surging particles.